1. Field of the Invention
The present invention relates generally to temperature management systems. More particularly, the present invention relates to systems for managing the temperature of discrete thermal components.
2. Description of the Related Art
To drill a well, a drill bit bores thousands of feet into the crust of the earth. The drill bit extends downward from a drilling platform on a string of pipe, commonly referred to as a “drill string.” The drill string may be jointed pipe or coiled tubing. At the lower, or distal, end of the drill string is a bottom hole assembly (BHA), which includes, among other components, the drill bit.
In order to obtain measurements and information from the downhole environment while drilling, the BHA includes electronic instrumentation. Various tools on the drill string, such as logging-while-drilling (LWD) tools and measurement-while-drilling (MWD) tools incorporate the instrumentation. Such tools on the drill string contain various electronic components incorporated as part of the BHA. These electronic components generally consist of computer chips, circuit boards, processors, data storage, power converters, and the like.
Downhole tools must be able to operate near the surface of the earth as well as many thousands of feet below the surface. Environmental temperatures tend to increase with depth during the drilling of the well. As the depth increases, the tools are subjected to a severe operating environment. For instance, downhole temperatures are generally high and may even exceed 200° C. In addition, pressures may exceed 20,000 psi. In addition to the high temperature and pressure, there is also vibration and shock stress associated with operating in the downhole environment, particularly during drilling operations.
The electronic components in the downhole tools also internally generate heat. For example, a typical wireline tool may dissipate over 100 watts of power, and a typical downhole tool on a drill string may dissipate over 10 watts of power. Although there is electrical power dissipated by a drill string tool, the heat from the drilling environment itself still makes internal heat dissipation a problem. The internally dissipated heat must be removed from the electronic components or thermal failure will occur.
While performing drilling operations, the tools on the drill string typically remain in the downhole environment for periods of several weeks. In other downhole applications, drill string electronics may remain in the downhole for as short as several hours to as long as one year. For example, to obtain downhole measurements, tools are lowered into the well on a wireline or a cable. These tools are commonly referred to as “wireline tools.” However, unlike in drilling applications, wireline tools generally remain in the downhole environment for less than twenty-four hours.
A problem with downhole tools is that when downhole temperatures exceed the temperature of the electronic components, the heat cannot naturally dissipate into the environment. The heat will accumulate internally within the electronic components unless there are provisions to remove the heat. Thus, two general heat sources must be accounted for in downhole tools, the surrounding downhole environment and the heat dissipated by the tool components, e.g., electronics components.
While the temperatures of the downhole environment may exceed 200° C., the electronic components are typically rated to operate at no more than 125° C. Thus, due to the extended time downhole, heat transfer from the downhole environment and the heat dissipated by the components will result in thermal failure of those components. Generally, thermally induced failure has two modes. First, the thermal stress on the components degrades their useful lifetime. Second, at some temperature, the electronics fail and the components stop operating.
Thermal failure is very expensive. The expense is not only due to the replacement costs of the failed electronic components, but also because electronic component failure interrupts downhole activities. Trips into the borehole also use costly rig time. An effective apparatus and method to cool electronic components in downhole tools would greatly reduce costs incurred during downhole operations associated with thermal failure.
A traditional method of cooling the electronics in a downhole tool involves modest environmental temperatures, such as may be found near the surface of the earth. Near the surface of the earth, the electronics operate at a temperature above the environmental temperature. In modest environments, the electronics are thermally connected to the tool housing. The thermal connection allows the heat to dissipate to the environment by the natural heat transfer of conduction, convection, and/or radiation. Temperature gradient cooling will only work, however, if the temperature gradient between the electronics and the environment is large enough to adequately cool the electronics.
A traditional method for reducing thermal failure in harsh thermal environments, such as thousands of feet below the surface of the earth, is to place the electronics on a chassis in an insulated vacuum flask. The vacuum flask acts as a thermal barrier to retard heat transfer from the downhole environment to the electronics. However, thermal flasks are passive systems that only slow the harmful effects of thermal failure. Because of the extended periods downhole in both wireline and drill string operations, insulated flasks do not provide sufficient thermal management for the electronic components for extended periods. Specifically, the flask does not remove the heat generated internally by the electronic components. Further, a thermal mass, such as a eutectic material, can be included in the flask to absorb heat from the downhole environment as well as the heat generated internally by the electronics. However, both the thermal flask and the thermal mass are only used to thermally manage the temperature of the interior of the electronics compartment. Because the discrete components are internally generating heat, they will remain at a higher temperature than the general interior of the electronics compartment. Thus, thermal failure continues to be a problem.
Another cooling method for deep-well cooling uses an active cooling system to cool electronics in a downhole tool. In this method, water in one tank is in thermal contact with the electronics chassis of the downhole tool. The water absorbs heat from the downhole environment and the electronics and begins to vaporize at 100° C. so long as the pressure of the tank is maintained at 1.01×105 Pa (14.7 psi). In order to maintain the pressure, the steam is removed from the tank and compressed in a second tank. However, sufficient steam must be removed from the first tank in order to maintain the pressure at 1.01×105 Pa. Otherwise, the boiling point of the water will rise and thus raise the temperature of the electronics chassis in the first tank.
In practice, active steam cooling has significant problems. First, this method has very large compression requirements because the compressed steam in the second tank cools to the temperature of the downhole environment. The compressor must be able to compress the steam to a pressure greater than the saturation pressure of steam at the temperature of the downhole environment, which is 1.55×106 Pa (225 psi) at 200° C. Second, this method is also time limited based on the amount of water in the first tank because when all the water in the first tank vaporizes, the cooling system will not function. In addition, the method does not isolate the electronic components but instead attempts to cool the entire electronics region. While the temperature of the region may remain at 100° C., the temperature of the discrete electronic components will be higher because they are internally generating heat. Consequently, this system does not effectively maintain the temperature of the discrete electronic components in order to minimize the effects of thermal failure.
Another cooling method attempts to resolve the problem of the high compression requirements of the above-mentioned cooling system by use of a sorbent cooling system. This method again uses the evaporation of a liquid that is in thermal contact with the electronic components to maintain the temperature of the components. Instead of using a compressor to remove the vapor, this method uses desiccants in the second tank to absorb the vapor as it evaporates in the first tank. However, the desiccants must absorb sufficient vapor in order to maintain a constant pressure in the first tank. Otherwise, the boiling point of the liquid will rise as the pressure in the lower tank rises.
Like the previous method, the sorbent cooling system also has significant problems. First, sorbent cooling only cools the entire electronics region, not the discrete electronic components. Thus, because of internal heat dissipation, the electronic components may remain at a higher temperature than the entire electronics region. Second, the desiccants must absorb sufficient vapor in order to maintain a constant temperature in the first tank. Otherwise, the liquid will evaporate at a higher temperature and thus the temperature in the first tank will increase. Further, the amount of water in the first tank limits the system. Once all the water evaporates, the system no longer functions.
Other methods also cool electronics apart from downhole applications. For example, micro-channel heat exchangers cool microprocessors and other microelectronic devices in surface-based applications. However, these systems operate in an environment where the ambient temperature is less than the device being cooled. In a downhole environment, the ambient temperature is often higher than the recommended operating temperature of the components being cooled. These methods will not function properly in a downhole environment because they cannot remove the heat from the components in an environment where the ambient temperature is higher than that of the components.
None of the known cooling methods effectively and efficiently controls the temperature of electronic components in downhole tools. An effective cooling system for electronic components in downhole tools is one that performs at least one or both of the following: (1) isolates thermally sensitive components from the environment; and (2) removes heat from thermally sensitive components. Consequently, to effectively manage the temperature of discrete thermal components in downhole tools, the present invention has been developed. Other objects and advantages of the invention will appear from the following description.